We have reported that cells producing RD114 virus contain two forms of reverse transcriptase; one is apparently identical to that found in RD114 virions while the other is larger. We have now found that both of these forms possess RNase H activity and are non-processive DNA polymerases. A number of replication-defective variants of MuLV have been isolated in our laboratory. Some of these variants are deficient in reverse transcriptase activity, while others show impaired processing of the gag gene precursors. When cells expressing Fv-1 restriction are infected with a single particle of restricted MuLV, they become transiently permissive to infection by other virus particles of the restricted tropism. This phenomenon, termed "abrogation" of Fv-1 restriction, requires the integrity of at least a portion of viral genomic RNA, and is apparently accompanied by a burst of early viral protein synthesis.